1. Field of the Invention
The present invention is directed to an apparatus for stirring glass, and in particular to an apparatus for stirring glass in transit from a main supply body to a working body or to a forming apparatus.
2. Technical Background
Formed glass is often considered to be a relatively inert material. Indeed, for this reason glass vessels often serve as containers in a vast array of different industries. However, during the glass manufacturing process molten glass is conveyed at very high temperature (in excess of 1600° C. in some cases). At such high temperatures molten glass itself can be quite corrosive, thus requiring a corrosion-resistant system of piping and containment. This corrosion can lead to failure of the vessel material. Consequently, most containment and transfer systems for molten glass rely upon vessels constructed from refractory metals. One such vessel is the stirring chamber.
In a typical glass manufacturing process, glass precursors, or batch materials, are combined and melted in a furnace to form molten glass (the “melt”). The glass stream flowing from the batch-melting tank or other vessel may vary in refractive index both longitudinally and transversely at any given time. Longitudinal variations generally result from changes in the batch and in the melting conditions; transverse variations generally result from volatilization of molten glass constituents and from corrosion or erosion of the melting-container refractories and present themselves in the form of cords or striae.
The presence of such variations is of no particular significance in the production of many types of glassware. When glass designed for ophthalmic or other optical purposes is being melted, however, the presence of such variations assumes primary importance since the quality and, hence, the saleability of the resulting ware are controlled thereby; and the reduction or substantial elimination of such variations becomes not only desirable but essential if satisfactory ware, i.e., ware in which the degree of homogeneity or variation of refractive index within an individual piece is maintained within a desired degree of tolerance, is to be produced.
By careful control of the batch composition together with maintaining substantially constant melting conditions, longitudinal variation of the refractive index can be held within a relatively narrow tolerance.
Through use of a homogenizing or stirring process cords or striae present in the glass can be substantially eliminated.
During the stirring process, the stirring apparatus stirs the molten glass and breaks the cord into increasingly finer divisions until what cord has not been homogenized into the melt is of inconsequential size.
As with the other molten glass conveying portions of the glass making process, the stirring apparatus, and in particular the rotating stirrer, is typically constructed from a refractory metal capable of withstanding the high temperature, corrosive environment of the molten glass. The refractory metal generally chosen for this application is typically platinum, or a platinum rhodium alloy.
In spite of its advantages, however, platinum, or alloys thereof, is not erosion proof, and the high stress developed within the stirring apparatus during the stirring of the viscous molten glass may lead to erosion of the metal surface and subsequent contamination of the molten glass with refractory metal particulate.
What is needed is a method of reducing erosion of the refractory metal surfaces of the stirring apparatus, and in particular, the rotating stirrer.